1. Field of the Invention
The present invention relates to a rear conversion lens to be mounted in rear of a primary lens, and more particularly to a compact rear conversion lens which can be mounted in a compact camera or video camera.
2. Related Background Art
A conversion lens to be mounted in front of or in rear of a main lens in order to expand a focal distance of the main lens has been known.
A so-called front conversion lens which is mounted in front of the main lens can readily expand the focal distance of the main lens but it leads to an increased diameter of the front conversion lens. Accordingly, it is difficult to mount it in a compact camera.
On the other hand, a so-called rear conversion lens (RCL) which is mounted in rear of the main lens is basically of a structure which is advantageous to compactness.
As a result, many proposals have been made, for example by Japanese Laid-Open Patent Applications No. 60-214327, No. 60-179712 and No. 58-195817 and Japanese Patent Publication No. 61-45207.
Although many efforts have been done in those known techniques, they are still not enough in the following aspects.
In the Laid-Open Patent Application No. 60-214327, a magnification is as low as 1.2 and effect as the RCL is small.
In the Laid-Open Patent Application No. 60-179712, the magnification of the RCL is 1.5-2.0 but the compactness of the structure is not satisfactory because it uses three lenses, positive, negative and negative lenses.
On the other hand, in the Laid-Open Patent Application No. 58-195817, the RCL comprises two lenses, positive and negative lenses but a magnification is as low as 1.15. Thus, an effect as the RCL is small.
In the Patent Publication No. 61-45207, a magnification of 1.5 is attained with two lenses, positive and negative lens but a correction for an outer coma aberration which is caused by the RCL is insufficient and a satisfactory performance is hard to attain.